Use the measured figures for the coefficient of friction and you will be closer to reality with your estimates. Several teams have posted measured figures on CD, and their measurements are roughly twice the inline published values.
I would also like to say that a driver who accelerates all the way down the field and then crashes into the back wall, or into another robot near the back wall, is not engaging in an accident. A reasonable expectation is that drivers will be required to attempt to maintain control of their robots, and will be expected to plan their acceleration and braking so that they arrive at their destination without a high speed crash. I would at least hope that this will be the case, although I have not yet run across this expectation spelled out as I read the rules. I will have to read the rules a little more closely, I guess...
Eugene
Quote:
Originally Posted by dtengineering
Okay... seeing as how my other car is still on this planet, I'm a bit hesitant to question these numbers, but I was doing some calculations with our programmers this evening to figure out peak velocities and such and have to question the assumed closure velocity cited here.
We used the published value of static coefficient (.06) of friction to determine that a 150 lb (68kg) robot would have a normal force of 668n and a peak forward force of 40N. The mass of the robot, plus trailer, is 186lb, or 84kg, giving a peak accelleration of 0.47 m/s/s
Next we assumed that the effective length of the playing field was 15m. Although 54 feet works out to be 16.5m, or thereabouts, the length of the robot and trailer, as well as the driver station bumpers must be subtracted from the space available for picking up speed.
Assuming constant acelleration, of .47m/s/s over 15m, it should take a minimum of 8 seconds to cross the playing field from one end to the other, with a peak impact velocity of 3.76 m/s or... 12.3 feet per second.
Now this is the peak velocity of a robot hitting the end... but it is also the maximum impact velocity that any two robots could sustain. If each started out at one end of the playing field, they would meet in the middle, and would each only have reached 6.15 fps each, for a closing velocity of 12.3 fps, which is just 2/3 of the assumed 18 fps velocity impact. (Actually it would be lower than 12.3fps, as the effective length of the playing field would again be diminished by the length of the second robot/trailer combo unit.)
That isn't to say that some robots might not exceed the published coefficient of friction as the playing field wears, or that a 12 fps impact is something to be laughed off without concern... we'll be building a solid robot and strapping solid bumpers on it... but the peak closure speed and resulting extreme G-forces didn't mesh with our calculations and we were wondering if we had somehow missed something.
Or, perhaps, if the 18fps impact velocity is based on actual testing of robots on regolith, then the published coefficients of friction don't provide an accurate estimation of robot performance. I know a few teams have posted suggesting that their experimental results for coefficients of friction are much higher than the published values.
Any suggestions?
Jason
<Edit> first assumption... that is not quite right. We assumed all of the weight of the trailer would be over the trailer wheels. Some of it will contribute to the normal force of the robot and thus improve traction and accelleration. Even assuming 100% of the trailer weight does so, however, peak accelleration is just .6 m/s/s and it takes 7 seconds to make the trip with a peak velocity of 14 fps. We're getting closer...
second assumption... we were assuming a straight line path from one end to the other... it may be possible to achieve a slightly higher peak velocity by taking a curved path along the playing surface... </edit>
|